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KAlSi3O8 A colorless, white, cream-yellow, flesh-reddish, or gray potassium feldspar that usually contains some sodium feldspar, either as albite or analbite or in some intermediate state; it is or appears to be monoclinic. Also known as common feldspar; orthose; pegmatolite.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a rock-forming mineral of the feldspar group with the chemical composition K[AlSi3O8]. Orthoclase contains admixtures of Na (up to 8 percent Na2O); less frequently, it contains Ba and small amounts of Fe, Ca, Rb, and Cs. It crystallizes in the monoclinic system to form prismatic crystals. Twin crystals are characteristic. Orthoclase, unlike microcline, exhibits perfect cleavage at a 90° angle (hence the name “orthoclase”). It is light pink, brownish yellow, or red. The mineral has a vitreous luster, a hardness on Mohs’ scale of 6–6.5, and a density of 2,550–2,580 kg/m3.

Orthoclase is one of the principal rock-forming minerals in magmatic rocks. Clusters of large orthoclase crystals occur in pegmatite veins. The mineral is often formed during regional and contact metamorphism. Orthoclase is used in the manufacture of glassware and ceramics.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Overlying these cherts, which appear to be local, there is a small succession of light coloured lavas which have not yet been studied, but which include orange coloured beds, probably due to a high orthoclase content.
Among the varied clasts in the conglomeratic deposits one finds angular fragment of orange coloured, orthoclase feldspar rich lava which must have been locally eroded.